1. Field of the Invention
The invention relates to liquid level sensors, and has especially advantageous application in a laboratory environment where small changes in liquid levels are to be measured.
2. Description of the Related Art
A wide variety of liquid level sensing methods have been proposed and employed. Mechanical methods have been employed wherein a buoyant element rides on the surface of the liquid and its position is detected. While simple to design and implement, these are difficult to miniaturize. The container has to be large enough to allow the floater to move up and down freely.
Acoustic methods have also been used in which an ultrasonic transmitter/receiver is used to launch a sound wave at the surface and detect its return. The beam spot of the sound wave is usually too large for the device to work reliably inside a small tube.
Pressure based sensing using a pressure transducer at the bottom of the container has been implemented wherein the liquid height is determined by measuring the pressure at the bottom of the container. These sensors do not work well for small volumes when surface tension becomes comparable to gravity.
Electrical schemes have also been devised. For example, the capacitance of a container may be measured to indicate fluid level. These types of detectors have been usually implemented as all-or-nothing type detectors, wherein the liquid rises to some pre-determined level, and the change is large enough to trip the detector. Another electrical scheme is described in U.S. Pat. No. 3,848,627 to Page. This device is intended to measure the water level in swimming pools, and includes a series of electrodes at different heights in a liquid filled tube. The lowermost electrode is grounded. A circuit is coupled to the remaining electrodes that energizes selected relay coils as the other electrodes change state between being grounded or electrically floating as the water level rises and falls.
None of these devices can detect small volumes at high resolution. Thus, an improved, reliable, and small size level sensor is needed.